1. Field of the Invention
The invention relates to an upsetting method for working a metal slug, a method for preparing a slug for a forging operation according to the method and a device for implementing the method.
2. Discussion of the Background
Forged metal parts are generally obtained by forging of slugs or billets which are raw blanks of semi-finished metal parts globally in the form of bars, used as basic elements for forming the part to be obtained by forging, their volume corresponding to the volume of the latter, increased by the lost volume during the forging. For example, in a turbojet engine, the fan disks or compressor drums are obtained by forging metal slugs.
In the aeronautical field, the safety criteria are restricting and impose checks at all the manufacturing steps. The slugs must notably be checked, for example with ultrasound, in order to detect the presence of inclusions or defects in the metal, which are at the origin of cracks during forging and possibly of failures of the finished part. In order to provide a satisfactory check with ultrasound, recent standards impose a relatively small section for the slugs, in the special case of the aeronautical field, of the order of 150 to 330 mm for nickel- or titanium-based metal slugs. If the finished parts of the turbojet engine are of a large volume, the slugs should therefore be of large slenderness, to compensate their small section.
These slugs, for which the length-over-section-diameter ratio, i.e., the slenderness, may initially be of the order of 12 for 1, should therefore be worked a plurality of times, in order to obtain slugs for which the slenderness in the special case is substantially equal to 3 for 1, a ratio for which it is possible to forge them, without their being contained sideways, without any risks of buckling or creation of defects in the fibers of the metal. The values given here correspond to nickel- or titanium-based metal slugs, with the rheology used for manufacturing parts of a turbojet engine. Working means hot deformation of a metal part in order to obtain an increase in its diameter and a reduction of its length, at an equal volume. Working is here obtained by upsetting, by placing the metal slug under pressure.
The prior art proposes upsetting devices for working metal slugs comprising two half-shells, each including a frustro-conical shaped housing. A slug is positioned in the lower half-shell, the two half-shells being pressed against each other by a press, in order to provide upsetting of the slug which therefore assumes the shape, here with a hexagonal longitudinal section, corresponding to the housing between both half-shells. Several upsetting operations are required for obtaining the slug which may be used in the forge.
As the primary slugs, i.e., the slugs as provided before the first upsetting, are of large slenderness, there is a risk of buckling during upsetting. It is therefore necessary to proceed with a large number of upsetting operations, the slenderness being only slightly reduced at each operation, in order to obtain a slug, to reduce the risk of buckling, without however canceling the latter. As the geometry of the shells is fixed, it is necessary to have as many upsetting devices as there are geometries of slugs, both from the point of view of their section and of their length; the number of required devices is therefore very large because of the different geometries and volumes of the parts of a turbojet engine on the one hand, of the necessity of carrying out a large number of upsetting operations on the other hand. Moreover, the dimensions of the devices from the prior art are large, because of the presence of two half-shells. The obtained slugs are already in the form of blanks, since they conform to the shape of both half-shells, which may be a nuisance for their forging; metal flashes formed at the contact between both half-shells should further be suppressed by machining. Finally, the heat losses during the upsetting operation are large, as the slug extends between both half-shells at a distance from one another and therefore is in contact with air.